Endodontic failures are caused by the proliferation of residual bacteria that are left behind within the root canal due to the complexity of the root canal system that makes complete debridement with instrumentation and irrigation alone almost impossible. Our long-term goal is to develop photodynamic therapy (PDT) as an adjunctive technique to eliminate residual root canal bacteria after instrumentation and irrigation with sodium hypochlorite (NaOCI). The specific hypothesis to be tested by this research is that visible light can achieve a rapid and complete killing of residual root canal microorganisms after their sensitization with a photoactive drug. The specific aims are to: 1. Develop mono- and multi-species biofilms with microorganisms that are common in endodontic failures and/or infections on the surface as well as in the dentinal tubules of root canal systems of extracted teeth. We will (i) characterize these biofilms by scanning electron microscopy and confocal scanning laser microscopy and we will (ii)analyze them using checkerboard DNA-DNA hybridization, polymerase chain reaction (PCR) assays and culture. 2. Investigate the photodestruction of root canal biofilms in vitro. We will (i) establish the parameters of light and photosensitizers for the complete elimination of root canal microbial biofilms, (ii)evaluate bacterial killing, and (iii)assess the distribution of cell killing within the biofilms. 3. Assess the safety of photodynamic therapy. We will (i) determine the viability of human periodontal ligament and bone cells after treatment, (ii)determine the therapeutic window whereby microorganisms can be inactivated without affecting host cell viability, and iii) investigate the induction of apoptosis. 4. Evaluate the antimicrobial effects of photodynamic therapy in human teeth ex vivo. We will use freshly extracted infected human teeth to evaluate the effect of photodynamic therapy on residual bacteria in the root canal system after complete instrumentation and irrigation with NaOCI. Relevance to public health: More than 2 million root canal retreatments due to residual microorganisms are performed yearly in the US. Photodynamic therapy may be an adjunctive procedure to kill residual root canal bacteria after their sensitization with a photoactive drug and their subsequent exposure to light.